On the surface of optical disk substrate, tracks are formed spirally or concentrically, and a number of sectors are defined on the tracks. The sector, when roughly divided, comprises a preformatted header portion, and a data portion in which information or other is written. At the outset of the data portion, there is provided a flag portion. The header portion is a place in which a physical lot number of the disk corresponding to the sector, that is, a track number, sector number or the like, has been written. When the optical disk substrate is molded, a row of pits is formed on this header portion, and no pit is formed in other portions and the data portion.
The formation of these sectors has heretofore been carried out according to CAV (Constant Angular Velocity) system formation. In this system, as shown in FIG. 6, the number of sectors laid out per circle is the same, either in the inner peripheral region of the disk or in the outer peripheral region of said disk. That is to say, although the memory capacity per sector is the same, the sector located in the outer peripheral region of the disk comes to have a longer recording area than the recording area of the sector located in the inner peripheral zone. Accordingly, this CAV system has such a drawback that the optical disk resulting therefrom has a relatively small total memory capacity.
As a way to solve such drawbacks as small memory capacity, there have recently been proposed as a sector formation system the zone formatted system, that is called MCAV (Modified Constant Angular Velocity) or ZCAV (Zoned Constant Angular Velocity) systems, and there is a growing tendency that the CAV system will give way to the zone formatted system in the near future. In this zone formatted system, the length of recording area per sector present is almost the same, either in the inner peripheral region of the disk or in the outer peripheral region, and the number of sectors per circle increases toward the outer peripheral region. On that account, the optical disks formatted according to the zone formatted system can have a memory capacity larger than that of the disks formatted according to the CAV system.
In this zone formatted system, a number of tracks are divided into groups so that each group defines a zone, as shown in FIG. 5. That is, the tracks are divided in the radial direction into groups so that each group has a predetermined number of tracks and defines one zone, and each track is divided into a predetermined number of sectors in the circumferential direction. The number of the sectors of each zone is increased one by one with the approach of the zone to the outer peripheral region. For example, in FIG. 5, there have been laid out 17 sectors per circle in the zone represented by the symbol a, 18 sectors per circle in the sector represented by the symbol b, and 19 sectors per circle in the sector represented by the symbol c.
As can be seen from the foregoing, the number of sectors which one zone includes per circle are different from that of sectors which the other zone adjacent to said one zone includes per circle. On that account, in the portion of FIG. 5 enclosed by a circle, next to the header portion of the sector of the final track of one zone is not laid out the header portion of the sector of the first track of the next zone, but is laid out the data portion of the sector of the first track of the next zone. For the sake of convenience of illustration, the enclosed portion of FIG. 5 is enlarged and shown in FIG. 3. In FIG. 3, the zones are assumed as a zone (m) and a zone (m+1), respectively, and the sectors laid out in the circumferential direction of these two neighboring zones are assumed as a sector (n) and a sector (n+1), respectively. As can be seen from FIG. 3, for example, next to the header portion of the final sector (n) of the track of the zone (m) is laid out the data portion of the first sector (n) of the track of the zone (m+1). Inversely, next to the header portion of the first sector (n+1) of the zone (m+1) is laid out the data portion of the final sector (n) of the track of the zone (m) .
In contract, in the case of CAV system, the sectors are equal in number, either in the inner periphery or in the outer periphery, as shown in FIG. 6, and hence the preformat portions of each sector are uniformly laid out straight in the radial direction.
Usually, optical disk substrates are molded by injection molding using a negative model known as a stamper fitted in a mold which is set to an injection molding machine. In practicing the injection molding of the optical disk substrate according to the zone formatted system, it is a matter of course that the pits to be transferred from the stamper to the header portion can be transferred and formed in the proper position. However, in the boundary portion between the zone and a zone of the outer peripheral side, the pits X1 formed in the header portion of the sector (n) of the zone (m) are sometimes transferred as a ghost Y1 (as shown by dotted line in FIG. 3) to the data portion of the zone (re+1). The circumstances under which this ghost takes place are extremely variable, and are roughly divided into the case (commonly known by pit shift) where the ghost Y1 is formed in the manner as if the pits X1 are dragged in the boundary and the case (commonly known as double transfer) where the ghost Y1 is formed independent of the X1.
The ghost Y1 takes place in one track or sometimes in several tracks. The pit shift or double transfer, which takes place in the boundary between the zones, is observed, as shown in FIG. 3, at the outer peripheral side (relation between X1 and Y1) or at the inner peripheral side (relation between X2 and Y2) of the zones. It is also known by experience that because of irregular formation of header portions of the sectors, the pit shift or double transfer is more apt to take place in the case of the zone formatted system in comparison with the case of CAV system. For the sake of convenience of illustration in FIG. 3, the pits X1 and X2, and the ghosts Y1 and Y2 are drawn much larger than life.
In such a case, because the header portions of each sector are in line in the radial direction in the CAV system, the pit shift or double transfer takes place in the adjacent header portion. On that account, even if the pit shift takes place in the CAV system, that is not much of a problem in most cases.
In the zone formatted system, however, the pit shift and double transfer do not take place in the header portions but in the data portions. Particularly, in the case of magnetooptical disks, a delicate rotation of a plane of polarization is sensed, thereby reading out data recorded in the disk. On that account, if the pit shift or double transfer takes place in the data portion, the plane of polarization is disturbed so that noises are carried on the reading signal, thereby bringing about occasional errors (burst errors) amounting to scores of bytes. According to the experiment conducted by the present inventor, the noises caused by the occurrence of the pit shift were larger than the real reading signals, and the burst errors took place. Whether the burst errors take place or not, or how large they become depends on the degree of the pit shift or double transfer and the performance of the disk drive used. In short, the recording regions in which the pit shift or double transfer took place are in a very unstable state.